Most of the epoxy resin systems that can be cured at temperatures below 100.degree. C. require the use of an aliphatic or alicyclic liquid polyamine or modified versions thereof. Compounds of this type are notable for their volatility and in the same cases require excessive care in handling due to skin irritation and/or other health hazards. In addition, the amines must be used in relatively low concentrations with liquid epoxy resins. Even with an intrinsically low viscosity such amines do little to reduce the viscosity of the highly viscous epoxy resin. Their usefulness is generally restricted to adhesives, and some surface coatings where solvents can be used to reduce viscosity.
Liquid anhydrides however, are low in viscosity having for example 50 to 100 cps. They are used in large concentrations such as 80 parts liquid anhydride per 100 parts of liquid epoxy resin, and when combined with epoxy resins yield systems of low viscosity which are ideally suited for filament winding and in the potting of electrical and electronic components. They require cure temperatures of over 100.degree. C., usually up to 150.degree. C., producing heat distortion temperatures of up to 130 C in the case of the most widely used anhydride, namely methyltetrahydrophthalic anhydride.
At the present time, in order to reduce cure time requirements, a temperature of 90.degree. C. is used for gelation. A solid quaternary ammonium salt is used as the promoter when methyltetrahydrophthalic anhydride is used with epoxy resin. This promoter produces a large exotherm during the curing with temperatures approaching 150.degree. C., at which temperature copious amounts of fumes are given off. Thus when filament wound parts are put into an oven to complete the cure, it is necessary to use sizeable exhaust ducts to get rid of these vapors. There is an incentive therefore to develop a system which can be cured under heat lamps at reduced temperatures to minimize this evolution of fumes.
In addition, when a liquid epoxy-anhydride system is gelled at elevated temperatures and assumes a solid form, it will undergo thermal shrinkage upon return to room temperature. Since the coefficient of thermal expansion is greater for epoxy resin than for glass or metals, when these materials are encapsulated in epoxy resin they will be subjected to a level of stress proportional to the gel temperature and the difference between that and ambient temperature. Hence the ability to cure at temperatures below 100.degree. C. would produce less strains in the epoxy resin composite.
The reaction between liquid epoxy resins and liquid anhydrides such as methyltetrahydrophthalic anhydride using state of the art promoters such as AP-6g produces well cured polymers only after one half hour gelation at about 90.degree. to 100.degree. C., followed by a two hour post cure at 150.degree. C. AP-6g is a 60% by weight solution of benzyltriethyl ammonium chloride in ethyleneglycol. The heat distortion temperature of this polymer is between 125.degree. and 130.degree. C. when the liquid epoxy resin is Epon 828 which is essentially the diglycidyl ether of Bisphenol A. The performance properties of this polymer in many cases will far surpass the requirements of the application in which it is used. In addition, most commercial users of this system are interested in faster cures at lower cure temperatures. For example, conduit for electrical application is currently made of polyvinyl chloride. When this is subjected to flame, highly toxic fumes are emitted and in confined areas such as subways or underground malls this constitutes an extremely hazardous environment. A filament wound epoxy fiberglass reinforced tube prepared from an epoxy-anhydride system would perform satisfactorily at the ambient surface temperatures with a neat resin system of relatively low heat distortion temperature such as 80.degree. to 90.degree. C., since polyester resins of this quality perform satisfactorily. However, styrene containing polyester resins present severe manufacturing problems with the new environmental restrictions concerning the presence of styrene monomer.
Resinous polyols, derived from the reaction of epoxides and polyols in the presence of benzyltriethyl ammonium chloride, are moderately viscous liquids that can be added to either portion of the two part epoxy-anhydride system. Since they contain a very reactive promoter for this reaction they are versatile additions to the list of promoters currently used for the epoxy anhydride reaction. Other liquid promoters available for this purpose include imidazole and imidazole adducts, benzyldimethylamine, and solutions of benzyltriethyl ammonium chloride referred to above as AP-6g. These must be added to the anhydride since they react with the liquid epoxy resin to form a homopolymer. Resinous polyols, derived from the reaction of a brominated epoxide resin with a polyol in the presence of benzyltriethyl ammonium chloride, may be used as additives for flame retardance. Resinous polyols derived from epoxy resins and long chain polyols such as LHT-240 in the presence of benzyltriethyl ammonium chloride may be used in a dual role to accelerate the reaction and to impart flexibility to the system. The polyether triol LHT-240 has a molecular weight of about 700. It is manufactured by Union Carbide at Danbury, CT.